The present invention relates, in general, to vehicle braking systems and, in particular, to a brake system for controlling both brake slip and traction slip.
German Patent Application DE 38 32 023 A1 describes a generic anti-lock-controlled brake system working in accordance with the recirculation principle. The pump is of the self-priming type in order to be able to supply pressure medium from the pressure medium storage reservoir through the brake line, via the tandem master cylinder, into the wheel brake of the spinning wheel for the purpose of traction slip control However, a vacuum must be prevented from forming in the wheel brake due to the self-priming action of the pump. To this end, a non-return valve opening in the direction of suction is built into the return line of the wheel brake which leads to the suction side of the pump, the pre-pressure of this non-return valve being at least as high as the atmospheric pressure.
If the brake circuit is arranged so that only the wheel brakes of driven and non-driven wheels are, at a particular time, in one brake circuit, (i.e. the so-called black-and-white arrangement) or an all-wheel drive, the problem is thus solved. The situation, however, is different with a brake system of the diagonal type when the vehicle has a one-axle drive. Then either brake circuit will operate on one brake of a driven wheel and one brake of a non-driven wheel at a time. In case of traction slip control, on the one hand, pressure should be built up only in the brake cylinders of driven wheels. On the other hand, it is also necessary to protect the brakes of the non-driven wheels from a vacuum. If, during a traction slip control operation, the pump primes through the brake line and the tandem master cylinder, it will generate a vacuum of about 0.8 bar. Depending on the throttling effects which appear between the reservoir and the connection of the suction line to the brake line, this vacuum may propagate, weakened, as far as the brake of the non-driven wheel if the shut-off valve is located between the feed lines to the wheel brakes of the driven wheel and of the non-driven wheel. However, if the two wheel brakes are located, relative to the master cylinder, beyond the shut-off valve, then the brake of the non-driven wheel would also be applied during traction slip control A complicated circuit logic, closing the corresponding inlet valve during traction slip control, is comparatively intricate and expensive If the shut off valve is arranged between the brake feed lines, closing of the inlet valve, as a rule, will be of no avail because, generally, non-return valves are connected in parallel to the inlet valves, with the non-return valves opening in the outlet directions and, thus, similarly permitting the formation of a vacuum.